Scalp blocks do not affect the accuracy of neuronavigation facial recognition registration

Purpose: Scalp block is a regional anesthesia technique to reduce the sympathetic response to skull pin application and postoperative pain in patients undergoing craniotomy. These blocks are often performed prior to surgical incision, however, the effect that these blocks have on neuronavigation facial tracing recognition accuracy is unclear because they may distort facial anatomy. Methods: A series of 25 patients undergoing supratentorial craniotomy were administered scalp blocks prior to surgical incision, and their effect on neuronavigation accuracy was assessed. Statistical analysis utilized a two-tailed matched t-test. Results: Bilateral supraorbital and auriculotemporal scalp blocks did not significantly affect the accuracy of facial recognition registration. Conclusion: Scalp block does not interfere with neuronavigation facial recognition accuracy during neurosurgical procedures.

The effect of scalp blocks on neuronavigation registration accuracy was tested in a series of 25 patients undergoing supratentorial craniotomy for tumor. After induction of anesthesia, patients had a neuronavigation registration performed, the supraorbital and auriculotemporal scalp blocks were performed, and then another neuronavigation registration session was again performed. The difference in accuracy was assessed statistically by way of a two-tailed matched t-test. In this series of patients, scalp blocks did not significantly affect the accuracy of facial recognition registration.
No competing interests were reported by any of the authors. No external funding sources were utilized.

Methods
Patients were screened preoperatively for contraindications for scalp block, including local anesthetic allergy, and bone defects which could cause intracranial spread of local anesthesia. Scalp blocks were administered at the discretion of the attending anesthesiologist assigned to the case, and were administered by one of two anesthesiologists in each case, (anesthesiologist A administered 13 patients scalp blocks, and anesthesiologist B injected 12 patients scalp blocks). A total volume of 10 mL of 0.5% bupivacaine was

Introduction
Scalp blocks are a regional anesthesia technique utilized to attenuate sensory nerve transmission from the scalp. They have been shown to reduce postoperative pain [1][2][3][4] as well as the sympathetic response to skull pin application [5][6][7]. The resultant injection produces a visible subcutaneous deposit of local anesthesia, and distorts the overlying tissue in proportion to the volume of local anesthetic injected. The application of these blocks are often performed prior to incision. Due to the tissue distortion produced by these scalp blocks, it is plausible that they could render neuronavigation facial tracing recognition software inaccurate. This possibility has to date not been scientifically tested. administered to each patient through a 27 ½ gauge needle, with each of the two supraorbital injections receiving 3 mL, and each of the auriculotemporal injections consisting of 2 mL. Patients were consented to the surgical procedure by the surgical team which included the use of neuronavigation. After the induction of anesthesia, the neuronavigation technicians registered the patient in the usual fashion using the Medtronic StealthStation™ S8 Surgical Navigation System (Medtronic, Minneapolis, Minnesota, United States). Success in navigation accuracy was achieved by meticulously tracing the patients face, nose, brow, mastoids, and scalp to collect accurate points for the registration.
The administration of this study was approved by the Stanford University Institutional Research Board (IRB 65,215).
The Medtronic system yields a single number Registration Accuracy Value (RAV) in millimeters (mm), with accuracy within 5 mm being the accepted margin of accuracy at the discretion of the surgical team. It is the indication of the amount of error in the registration. Our institutional standard is within 2 mm on most cases, however. When the accuracy of the registration was completed by the neuronavigation technician, an additional registration run was undertaken by a senior neurosurgical trainee under the constant supervision of the neuronavigation technician. Bilateral scalp blocks were injected at the supraorbital and auriculotemporal nerve locations between the two registration runs. RAV were then compared using a two-tailed matched t-test to determine if the scalp blocks impacted accuracy.

Results
The results from the 25 subjects are shown in Table 1. Analysis of this data using a two-tailed matched t-test did not reveal a significant difference (p = 0.5795). This data shows that there was not a significant difference in accuracy after scalp blocks had been infiltrated in the RAV.

Discussion
Scalp blocks have been demonstrated to reduce post-operative pain [1][2][3][4] as well as sympathetic response to skull pin fixation [5][6][7]. Administration of these blocks involves infiltration of local anesthetic around sensory nerves that carry pain signals from the scalp, such as the supraorbital and auriculotemporal nerves. The application of local anesthesia can cause significant soft tissue distortion. This distortion was postulated to potentially reduce the RAV. However, scalp blocks administered bilaterally in the supraorbital and auriculotempral locations did not impact the RAV in a statistically significant way.
The administration of scalp blocks did not affect the registration process primarily because the RAV is determined by the relationship of the aforementioned bony landmarks of the face and scalp regions to the intracranial anatomy. The distortion of the soft tissues with the scalp block injection therefore did not significantly alter these anatomical landmarks. However, the small differences in individual RAV between patients could be explained by small amounts of local anesthesia tracking over top of the bony landmarks needed for registration. As can be seen in Table 1, some patients had decreased RAV after scalp block, although this was in most cases just fractions of a millimeter. Curiously, in some patients, RAV increased after scalp block administration. This may be explained by the scalp block local anesthesia tissue distortion tracking over top of bony landmarks needed for registration, but the computer software identifying these regions as outliers and excluding them from the registration algorithm. The exclusion of these areas, then, may have increased the overall accuracy of the registration process.
Anesthesiologists and neurosurgeons who wish to inject scalp blocks prior to neuronavigation registration would therefore be advised to review which landmarks are needed for registration with the operator of the neuronavigation system to avoid depositing local anesthesia adjacent to these critical areas.

Conclusion
Scalp blocks were not shown to have a negative impact on the RAV on the Medtronic StealthStation™ S8 Surgical Navigation System. Therefore, scalp blocks at the supraorbital and auriculotemporal locations may be injected prior to registration using this navigation system without compromising the RAV as long as bony landmarks needed for accurate navigation registration are avoided.